In assembling composite sets of interconnections surrounded by insulators, the art has used a “Z-connection” method in which layers of conducting materials are connected using an electrically conductive paste.
Typically, as shown in FIGS. 4A and 4B, a set of dielectric sheets 460 and 465 having metal connections embedded in them have a set of vertical visa and horizontal metal lines or sheets. The horizontal lines or sheets 10 are placed between two sheets of dielectric 15 that are bonded in a sandwich. (Layer 10 is optional.) Holes or apertures between the metal sheets provide vertical connections. Fixed metal visa 420 formed by plating are used together with visa 436 formed by forcing a conductive paste made of epoxy or other plastic filled with a metal powder into a hole. Top and bottom layers 410 are made of copper or other metal. The metal is etched to define contact pads at locations 402 and 404 on which solder bumps are placed The assemblage is bonded together, by a lamination process performed at elevated temperatures and pressures such that the dialectic materials stick to each other and to the metal. The assemblage in FIG. 4A is generic and the one in FIG. 4B is customized. On the right of FIG. 4B, denoted with dotted line 442, a set of vertical members make a vertical path between upper and lower contacts 402 and 404. On the left in FIG. 4B, denoted by a dotted line 444, a conductive paste via 436 extends downwardly from a solder ball in the upper left of the Figure. A half-via 420′ connects via 436 to a horizontal line 10, at the end of which a second half-via 420′ makes contact with the next via 436. Two more visa carry the path down to the lower solder ball.
Those skilled in the art will appreciate that use of conductive paste gives rise to various problems: the paste is messy—it squeezes out of holes and can cause short circuits unless carefully cleaned up.
The paste is thick and often does not fill a hole properly, especially blind holes, causing an open circuit or higher resistivity than specified.
Paste particle size can be a problem in filling small holes. Pastes containing silver are expensive.
These problems are addressed by the present invention.